In coke oven plants, it is common to utilize ammonia for the removal of sulfur dioxide and other acid components from waste gases, the removal process producing solutions containing ammonium compounds such as ammonium sulfate or ammonium sulfite. In order to recover the ammonia from the ammonium sulfate or ammonium sulfite compounds, the solution is usually mixed with milk of lime (i.e., an aqueous solution of Ca(OH).sub.2) whereby the ammonia is substituted by the calcium. The treated solution is then fed into settling tanks where a sludge consisting of calcium precipitation products is separated. These precipitation products are difficultly soluble in water; but the settling time takes several hours. After settling of the calcium precipitation products, the clear liquid containing the liberated ammonia is usually fed to an ammonia distillation column. The resulting lime sludge consisting of calcium precipitation products is very difficult to remove from the settling tank. The sludge contains at least 50% moisture and, hence, a relatively large amount of dissolved ammonia; however gasification of such sludge (e.g., with water vapor) is expensive since the process must be carried out in closed tanks. Furthermore, problems are encountered in dumping such sludges.
Liquids from coke oven plants contain free and fixed ammonia in addition to other substances. In the past, it has been common to treat such liquids in two stages by stripping the free ammonia in a first stripping stage with vapors from the second stage. The coke oven liquid is fed to the top of a column or scrubber and flows down through a number of exchange trays, the vapor from the second column being introduced from below. In this manner, the free ammonia and other components which are not fixed are distilled off.
To remove the fixed ammonia from the liquid, milk of lime is usually added to the discharge from the first stripping stage. Substitution of the ammonia by calcium causes the ammonia to be liberated, this liberated ammonia being distilled off with water vapor as in the first column. The bottom discharge from the second or lime column contains the lime sludge, calcium sulfate, calcium sulfite, calcium carbonate and other calcium compounds. In the past, this sludge was then separated in subsequent settling tanks and then usually dumped as explained above. Dumping of this material, however, is usually not now permitted under the existing environmental legislation.
Another disadvantage of the prior art process described above is that the lime column has to be shut down at intervals of several months, depending upon the composition of the liquid from the coke oven plant. This downtime is necessary since the calcium reaction products in some cases form stoney deposits in the column, particularly on the exchange trays; and these deposits must be removed manually. Attempts have been made to avoid such incrustations by the addition of inhibitors, but the cost of suitable inhibitors is very high. Furthermore, while inhibitors may delay the formation of deposits, they do not prevent the formation of such deposits which eventually must be removed manually.
In another prior art process, the liquid from a coke oven plant is mixed with lime in a separate vessel before entering the stripping column. In this separate vessel, the reaction product is allowed to settle and the discharge of clear liquid is then subjected to ammonia stripping in a known manner. A disadvantage of the process is that the long settling time required entails very large closed tanks. In addition, the ammonia not passing over into the vapor phase is entrained in the moisture in the settled sludge; and this makes it extremely difficult to remove, transport and dump the sludge. While it is possible to gasify the sludge with, for example, water vapor before removal from the vessel, this process is laborious and also expensive.